Not all compost tests are the same - TPS Lab offers two different tests for the essential qualities of compost:

Active Organic Matter - This measures the amount of organic matter that has been decomposed or digested by microbes into useable material by plants.

Loss On Ignition (LOI) measures the total organic matter content that is useful indetermining how much raw organic matter is available for microbial food to digest into plant-available nutrients.

U.S. Composting Council

TPS Lab is the 11th approved US Composting Council Seal of Testing Assurance ( USCC STA ) laboratory for all participating STA composters in the United States and Canada! You can find us listed at http://compostingcouncil.org/labs/.


Compost Testing and Organic Mix Testing

Composting testing is important for characterizing the beneficial physical and chemical properties of a compost as well as for identifying potential problems with compost use. TPS Lab employs testing procedures from the U.S. Compost Council’s Test Methods for the Examination of Composting and Composts as well as some proprietary testing methodology that is also essential to measure the quality of your compost.

For example, TPS Lab measures the humus fraction of the compost as well as the total organic matter to verify that the compost is fully mature. An indicator of compost maturity is high OM (humus) and Nitrate (NO3) > 30 ppm which indicates well matured (cured) compost by biologically active microbes releasing Nitrate (NO3). High OM (humus) and Nitrate (NO3) < 10 ppm indicates a very active to moderately active (curing stage) compost. Adjusting C : N ratio with fertilizer N or biologically with energy (molasses, fish, vegetable meals, etc.) will aid maturity. Just like a conventional fertilizer bag that comes with a guaranteed analysis and a label rate, we must also know the guaranteed analysis of composts to make sure that we apply safe rates to avoid accumulation of excessive mineral salts over time.

See Explanation of Soil Organic Matter here.

General Guidelines of Compost Testing and Organic Mix Testing

pH is a measure of active acidity in the feedstock or compost. Most finished composts will have pH values in the range of 5.0 to 8.5. Ideal pH depends on compost use. A lower pH is preferred for certain ornamental plants while a neutral pH is suitable for most applications. pH is not a measure of the total acidity or alkalinity and cannot be used to predict the compost effect on soil pH.

Soluble salts are determined by measuring Electrical Conductivity (EC) in a 1:5 (compost:water, weight ratio) slurry. Electrical Conductivity (EC) is related to the total soluble salts dissolved in the slurry and is measured in units of millimhos/cm (mmho/cm). Composts typically range from 1 to 10 mmhos/cm. High salinity may be toxic to plants. Ideal soluble salt levels will depend on the end use of the compost. Final compost blends with soil or container media/potting mixes should be tested for soluble salts.

The ideal moisture content for composting will depend on the water holding capacity of the materials being composted. In general, high organic matter materials have a high water holding capacity and a higher ideal moisture content. A typical starting compost mix will have an ideal 65-45 % moisture. Finished compost should have a 50-40 % moisture.


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